Fiber optic sensors have been used for a number of years to make measurements of physical parameters such as pressure, tensile or compressive forces, temperature, flow rate, humidity, refractive index and the like, particularly in hostile environments and long-duration monitoring since the optical properties and behaviors of fiber optic cables and sensors such as Bragg gratings are well-known, the technology of the optics is mature and the optical cables and sensors are particularly robust and resistant to damage or aging in the environments in which they are employed. Further, robust hardware structures can generally be easily adapted to convert the effects of the physical parameter of interest into a physical deformation due to a mechanical force, referred to as strain, that is variable with the physical parameter of interest to change the optical behavior of a portion of fiber optic cable or an optical sensor in a highly predictable manner, allowing measurements to be made at remote and inaccessible locations with a high degree of accuracy.
However, at the present state of the art, the amount of strain that can be used for making measurement is limited by the nature of the materials which can perform as fiber optic cables and sensors. Specifically, the glass or hard plastic materials that have been traditionally used have a large value of their Young's modulus of elasticity (hereinafter sometimes simply “modulus”) and very short elongation before fracturing which complicates making measurements involving large magnitudes of strain. Moreover, known methods of making sensors such as Bragg gratings, which are well known and understood and used for optical notch filters, optical multiplexers and demultiplexers or optical add-drop multiplexers, are complicated and expensive as well as resulting in sensors which are better adapted to relatively small strain dimensions. For example, typical (e.g., short period) fiber Bragg grating (FBG) structures have periodic gratings of less than one micron and are typically fabricated by excimer lasers and phase masks. Longer period FBGs have a grating periodicity in the range of 100 microns to 1 millimeter and are fabricated by ultraviolet (UV) radiation, ion implantation, femtosecond infrared (IR) radiation, carbon dioxide laser irradiation or diffusion of dopants into the core of a fiber optic cable which are expensive and complex with often relatively small process parameter windows.
These factors generally limit usable strain ranges to 5% or less of the sensor dimensions while large strain measurements are a critical problem for many applications such as monitoring the structures of aircraft, ships, buildings and other large structures and constructions that are subject to aging, positional shifting, or deformation by applied forces or other conditions of environment and/or use as well as for monitoring any other aspects of structural health. Moreover, the parameters of interest in such measurements may favor measurement over a substantial continuous distance such as for measurement of settling or subsidence or dimensional creep of materials under substantial force for an extended period of time which may require large sensor physical size whereas typical fiber optic sensors are very small and their cost of manufacture is generally proportional to their length. Therefore, fiber optical measurement over significant distances has been limited to measurement of distances between sensors (e.g. forming a Fabry-Perot cavity therebetween); requiring a costly and complicated arrangement that, in turn, requires optical measurements of extremely high precision, such as reflection return time or interference pattern detection and analysis rather than much simpler techniques of measurement of light intensity or spectrum.